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David,
Load up Canon's EOS Utility to control your camera to take pictures.
You want to take images of an evenly illuminated surface. Doesn't really matter how far, but, you want the light source to evenly illuminate the sensor. You also have to make sure that your camera's orientation doesn't change from when you captured your actual object. If the rotation of the camera, or focus, changes, then the flat frames will be invalid. Reason being, we wish to capture anomalies in the imaging train as at the time of object capture.
What you ideally want to capture is between 16 and 25 flat light frames. You will need to set your ISO to the lowest native setting of your camera. For Canon systems, this value is ISO-100.
You will need to experiment with exposure duration, but, what you want is to see a peak in your image's histogram, about 1/3rd of the way across the X-axis.
This might be a value like 1/30th of a second all the way up to a couple of seconds, depending on the intensity of your light source and aperture.
Once you've captured the flat lights, you need to capture your flat darks. Simply put your telescope cover on or lens cap, if imaging with a lens, and capture another 16-25 images at the same exposure duration as your flat lights. Your flat darks can be taken with the camera removed from the scope.
You will then median combine them in your software of choice.
Hope this helps.
H
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